1,3-Propanediol is a raw material of high-quality polyester fibers, including polytrimethylene terephthalate, and the demand therefor has thus been increasing in recent years. An example of a method for synthesis of 1,3-propanediol is a method of hydration and hydrogenation of acrolein described in Non-Patent Document 1. This method involves air oxidation of propylene, which is a petroleum raw material, in the presence of a catalyst and hydration and hydrogenation of acrolein resulting therefrom. This technique has been established as an industrial production method. However, development of a method for synthesizing 1,3-propanediol from biomaterials has been desired because of an increase in crude oil prices in recent years.
The present inventors have been conducting research aimed at development of a process for synthesizing acrolein, which is a precursor for 1,3-propanediol, from a glycerin waste product obtained, as a by-product, in the process for biodiesel production.
As described in Patent Document 1, a biodiesel is generally produced by a method involving the use of an alkaline catalyst. According to this method, a triglyceride (e.g., rape-seed oil) is subjected to transesterification in the presence of an alkaline catalyst (e.g., potassium methoxide) to produce a biodiesel. As a result of such reaction, a glycerin molecule is generated from a triglyceride molecule. Because of the inclusion of an alkaline catalyst in glycerin, no cost-effective process for recycling is available. At present, accordingly, most manufacturing facilities are required to incur incineration disposal waste costs.
Meanwhile, Non-Patent Document 2 describes a method for synthesizing acrolein from glycerin with the use of supercritical water. According to this method, an aqueous solution of glycerin as a biomaterial is mixed with high-temperature supercritical water at 35 MPa, the solution is instantaneously heated to 400° C., and acrolein is then synthesized. This method is characterized in that protons of a sulfuric acid, which was added in a trace amount to an aqueous solution of glycerin, function as catalysts that accelerate the dehydration of glycerin. If a glycerin waste product generated in the process of biodiesel production is directly treated with supercritical water, however, pipe clogging may disadvantageously occur. This may occur because an alkaline catalyst contained in a glycerin waste product is precipitated in supercritical water with low permittivity. This necessitates the removal of alkali metals from glycerin obtained, as a by-product, in the process of biodiesel production.
Patent Document 2 describes a method for purification of glycerin involving ion exchange. According to this method, a glycerin waste product containing impurities such as organic fatty acid and alkali generated in the process of biodiesel production is allowed to pass through ion-exchange resin to remove such impurities, and glycerin is thus purified. The glycerin waste product contains alkali metals in an amount of approximately 4% by weight and organic fatty acids in an amount of approximately 10% by weight or more, in terms of ion concentration. When the glycerin waste product is treated without pretreatment, accordingly, the time until the ion-exchange resin reaches the breakthrough point is shortened, recycling frequency is increased, and, in turn, costs for purification are increased disadvantageously.
Patent Document 3 describes a method for purification of glycerin involving distillation (FIG. 1). According to this method, glycerin generated at the time of transesterification of fats with methanol in the presence of an alkaline catalyst is purified. This process is characterized in that hydrochloric acid is added to an alkali-containing glycerin solution to precipitate an alkali metal chloride, and the chloride is roughly separated via filtration, and the product is then purified via distillation. Since this process involves purification via distillation, a large quantity of energy is required to reheat raw materials, which leads to an increase in costs for glycerin purification.